Problem description: In case it an Item value widget is listening items from another widget, and this broadcaster widget gets deleted, then the following runtime error is observed in php log after opening this Item value widget configuration form:

pg_query(): Query failed: ERROR:  invalid input syntax for type bigint: ""
LINE 1: ...lue_type FROM items i WHERE i.flags IN (0,4) AND i.itemid=''
                                                                     ^ [jsrpc.php:872 -> CApiWrapper->__call() -> CFrontendApiWrapper->callMethod() -> CApiWrapper->callMethod() -> CFrontendApiWrapper->callClientMethod() -> CLocalApiClient->callMethod() -> CItem->get() -> DBselect() -> pg_query() -> CConfigFile->{closure}() in include/db.inc.php:249]

Steps to reproduce:

1. Create a new dashboard on a fresh installation of zabbix
2. Add the following widgets to this dashboard:
   1. An "Item navigator" widget, that allows to select "Available memory" items
   2. An "Item value" widget, that listens to items that are selected on the above "Item navigator" widget.
3. Wait for the "Available memory" item on Zabbix server to get some data
4. Open created dashboard in edit mode
5. Delete the previously created "Item navigator" widget
6. Open "Item value" widget configuration form
7. Check for runtime errors

Result: the previously mentioned runtime error is observed.
Expected: no runtime errors should be observed.